The shoulder joint is a ball-and-socket joint with unique features that allow for exceptional freedom of movement. The hemispherical head of the humerus and the glenoid capsule of the scapula support the articular surfaces of the shoulder joint. The head of the humerus is significantly large relative to the shallow glenoid cavity. In addition, ligaments in the shoulder act largely to limit the degree of movement allowed in the joint: They do not act to maintain apposition of the joint surfaces. As a consequence of these and other special characteristics, the shoulder joint exhibits every variety of movement: flexion, extension, abduction, adduction, circumduction, and rotation. The range of movements comes as some cost to joint stability, however.
Shoulder instability and other maladies of the shoulder joint, such as arthrosis or fracture, can be sufficiently acute that prosthetic replacement of compromised joint features may be indicated. Replacement of the humeral head involves resecting the humeral head from the humerus and installing a humeral prosthetic at the resection.
Early shoulder prostheses attempted to mimic the upper portion of the humerus and extending to include the humeral head. They typically were unitary structures that included a stem to be anchored in the humeral canal and a hemispherical head to be positioned within the glenoid cavity of the scapula.
Later developments allowed for adjustments to the geometry of the prostheses. Differences in patient anatomy and surgical techniques necessitated maintaining large inventories of the early, unitary prostheses. Prostheses were kept on-hand with heads and stems of different sizes and various relative tilt angles and radial offsets.
The more-recently devised modular prostheses generally are modular systems. Their modularity allows flexibility with respect to either the tilt angle or the radial offset between the head and stem. Although some of these prior art modular systems utilize either a “standard” head or a “standard” stem, most still require a plurality of either the heads or the stems to provide complete tilt angle and radial offset flexibility. None of the prior art systems provides complete tilt angle and radial offset flexibility without requiring different modular head or stem components of each given size. As a result, substantial inventories are maintained of either the stems or heads, which are the most expensive components. Moreover, most of the known systems provide incomplete adjustability of prosthetic geometry.
FIG. 1 illustrates a modular humeral-prosthesis 1 disclosed in DE 19509037 to Habermeyer. The humeral-prosthesis 1 allows for adjustment of radial offset, inclination angle, and version (anteversion/retroversion). Humeral-prosthesis 1 includes a stem-module 3 that features a shank 5 having an upper-shank portion 7 and a tongue/tab 9 that supports a pin 11. Pin 11 hinges an angle-adapter 13 to the rest of the stem-module 3. Angle-adapter 13 fits over tongue/tab 9 and pivots on pin 11 through an inclination angle a as shown in FIG. 2. The angle-adapter 13 can be locked in place to retain a desired inclination angle.
Humeral-prosthesis 1 also includes a coupling adapter 17, shown in FIGS. 1-4. The coupling adapter 17 is shown in FIG. 3 separated from other prosthetic components. The coupling adapter 17 includes an adapter plate 27. A male Morse taper 29 extends from one side of adapter plate 27. A ball joint 31 extends from the adapter plate 27 on the side opposite the male Morse taper 29. The ball joint 31 is located eccentrically on the adapter plate 27. The eccentricity of the ball joint 31 on the adapter plate 27 allows for adjustment of a radial offset between the prosthetic stem 3 and a spherical cap 27 secured to the male Morse taper 29, as described further below. Once established, the radial offset is fixed using set screw 23 (FIG. 4).
Referring to FIG. 5, adjustment of anteversion/retroversion is provided by the angle-adaptor 13. As discussed above, the angle adaptor 13 pivots on the axis 11 at the top end 7 of the stem 3 to adjust the inclination angle a. In the view of FIG. 5 it can be seen that the angle adapter 13 is beveled at its interface with coupling adapter 17. The beveling allows the adapter 13, and hence the stem 3, to pivot by way of ball joint 31 with respect to the coupling adapter 17. The stem 3 and coupling adapter 17 rock through an angle b to one face 15 or the other of coupling adapter 17. Faces 15 on coupling adapter 17 act as stops to define the maximum pivot to either side. Set screw 23, used to retain the desired radial offset, also fixes the desired version.
A need exists in the prior art for a modular shoulder prosthesis that features universal setting of radial-offset, inclination angle, and anteversion/retroversion, with independent fixing of each setting.